This invention relates to a method and apparatus for programmable, periodic rotation of a rotatable member and particularly, to periodic rotation of the rotor assembly of a turbogenerator during the time that it is not rotated in its normal manner for generating power.
It is known in the art that it is beneficial to rotate a rotor of a machine, particularly a massive rotor which is subjected to thermal gradients during and following normal operation. This rotational function is begun after the rotor has been at rest for a relatively short period of time in order to prevent distortion of the rotor shaft. See, for example, U.S. Pat. Nos. 4,018,094; 4,090,409; 4,267,740 and 4,643,637. However, the turbo-generator rotor, when rotated continuously at low speed during cooldown or for other assignments, incurs possible fretting corrosion on the generator rotor, and low cycle fatigue stresses on the low pressure turbine rotor.
It is sometimes necessary to position the rotor shaft accurately for maintenance purposes, such as blade or seal inspection, rotor balancing activity or rotor positioning at start-up of the rotor for normal operation. If the rotor shaft is turned by an electric motor, as in U.S. Pat. No. 4,643,637, the motor is frequently turned on and off in a short time span, or "jogged", to accomplish the positioning of the rotor shaft. The positioning of the rotor shaft by such method depends on many things, including the skill of the operator, and the electric motor is not only strained by such activity but also has a shorter life. To reduce such problems, it has sometimes been the practice not to use the electric motor for jogging purposes, and instead, to wrap the rotor shaft with a cable and to pull the cable with a hoist.
Ratchet type rotor shaft rotating devices of the type disclosed in some of the patents identified hereinbefore utilize a ratchet mechanism subject to ratchet wheel tooth breakage and pawl problems where then it is necessary to dismantle the machine of which the rotor shaft forms a part for repair, or to forego the turning of the rotor shaft. Further, rotor balance problems can develop from such incidences.